System for determining stretch characteristics of thermoplastic articles

ABSTRACT

Disclosed are a method and apparatus for determining the stretching characteristics of thermoplastic articles such as bottles on a consistently accurate basis by making an initial measurement such as dimension or volume, stretching the article to the onset of strain hardening, and correlating the measurements to a physical property such as inherent viscosity, intrinsic viscosity or molecular weight of the thermoplastic article.

This is a continuation application of application Ser. No. 07/964,226filed on Oct. 21, 1992, now U.S. Pat. No. 5,365,792.

1. TECHNICAL FIELD

This invention relates in general to a method of determining thestretching characteristics of thermoplastic articles having apredetermined relation of natural stretch ratio to a selected physicalproperty. More specifically, this invention relates to a method ofdetermining the stretching characteristics of thermoplastic articlessuch as bottles on a consistently accurate basis by making an initialmeasurement such as dimension or volume, stretching the article to theonset of strain hardening, and correlating the measurements to aphysical property such as inherent viscosity, intrinsic viscosity ormolecular weight of the thermoplastic article.

2. BACKGROUND OF THE INVENTION

While the discussion herein will be, for the most part, directed tobottles made of polyethylene terephthalate (PET) polymers andcopolymers, it should be understood that the invention has wideapplication in thermoplastic articles of various polymers. However, adiscussion of the invention as it pertains to such bottles is adequatefor a full understanding thereof. Also, it is well known that inherentviscosity (IhV), as well as intrinsic viscosity, are often used in theart as an indication of molecular weight. Hence, IhV, will be used forthe most part in describing the present invention herein. IhV ismeasured at 25° C. using 0.50 gram polymer per 100 mL of a solventconsisting of 60% by weight phenol and 40% by weight tetrachloroethane.

Tg (glass transition temperature) is measured using conventional DSC(differential scanning colorimetry) techniques.

When producing beverage bottles from polymers such as PET, it isimportant that the polymer be well oriented during stretching. Properorientation results in uniform material distribution in most areas ofthe bottle. Some portions of the bottle, such as the threads, supportring, and center region of the base, are not oriented. Attaining theproper orientation is dependant upon the IhV. If the IhV is too low, thebottle will have thin sidewalls which causes a reduction in theshelf-life of the beverage, and the bottle will also expand excessivelydue to internal pressure.

It should be noted that while the stretching characteristics of PET arehighly dependant upon IhV, other factors also have an effect, e.g., thePET's temperature when stretching occurs, the PET's level ofcopolymerization, moisture content, amount of free volume relaxation,and rate of stretching have an effect on the amount that PET willstretch. However, if these variables are constant for all tests,comparative results are achieved.

Bottles are generally made by first injection molding a preform, whichis then blown into a bottle. It is not practical for PET bottleproducers to check. bottle preform IhV in their plants. IhV measurementsare expensive and can be rather erratic unless the test is verycarefully controlled. Therefore, IhV testing is not common in the bottleindustry. As a result, most bottle producers often do not learn of IhVproblems until bottles are blown (typically 1 to 14 days after molding).

It has now been discovered that IhV can be determined on a relativebasis by relating it to the natural stretch ratio (NSR) of articles tobe tested. That is, if the NSR of thermoplastic bottles is alwaysdetermined under the same conditions for different samples, the NSR willindicate the IhV (or intrinsic viscosity or molecular weight). In thisway, for example, if an IhV of 0.72 is desired, the NSR of that articlecan be determined under given conditions (control). A graph for NSRplotted against IhV can be developed. Thermoplastic articles can then bestretched, either unilaterally or bilaterally, under the same conditionsas the "control". If this ratio is the same as for the control, it willindicate the same IhV. Or, if this ratio is different, the IhV can bedetermined by referring to the graph.

It is believed that the present invention may be used by bottleproducers to free-blow bottles and then plot bottle volume onstatistical process control charts as an indication of IhV.

Free-blowing of PET preforms is a well known technique used to obtainempirical data on the stretching characteristics of a particular PETformulation. Such data are used to design a preform for that formulationwhich will yield the desired bottle properties. Free-blow involvesheating the bottle preform to a temperature above its Tg and thenblowing it without a mold such that it is free to expand withoutrestriction until the onset of strain hardening is reached. In PETarticles, it will be apparent that the onset of strain hardening hasbeen reached when pearlescence (caused by microcracks) appear. Thus, thefree-blow conditions (heating time and blow pressure) are adjusted sothat the free-blown bottle will exhibit a slight amount of pearlescence.

Once a particular design is put into commercial use, it is importantthat the preform IhV be maintained at a target design IhV, or areasonable variation thereof. If the preform IhV is not near the targetIhV, problems will occur during the bottle blowing process. Typically,IhV variations of plus or minus 0.03 dl/g can be tolerated.

A basic problem has existed in the past in consistently heating preformsto the same temperature during each free-blow test to consistentlyobtain accurate results.

BASIC DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view of apparatus suitable for carrying out thepresent invention.

FIG. 2 is a graph showing a typical stress-strain curve forthermoplastic articles.

FIG. 3 is a graph illustrating bottle volume vs. bottle IhV.

DESCRIPTION OF THE INVENTION

In general, the present invention provides a method of analyzing samplesof thermoplastic articles for a selected physical property in a mannersuch that consistent results are obtained which comprises

a) heating the articles by at least partially submerging them in aboiling liquid until at least the glass transition temperature isreached over the submerged portion,

b) stretching the articles at a temperature of at least their glasstransition temperature until the onset of strain hardening (OSH) occurs,and

c) determining a desired property which is, or is related to, theelongation of said articles.

As a practical application, this invention provides a convenient methodfor insuring consistency of wall thicknesses of thermoplastic bottles,which is of course necessary for quality control reasons. For example,if a bottle is blown from a preform having the correct molecular weight,that bottle can be used as a control for others by determining itsvolume after free-blowing. All other samples having the correctmolecular weight polymer should in turn have the same volume afterfree-blowing. For comparable results, however, the temperature atblowing must be consistently the same for all samples. It is difficultto insure the same temperature using conventional heating methods.Applicant has found that these temperatures can be held consistently thesame if the heat is supplied from boiling liquid, preferably water.Natural stretch ratios at that temperature (at, or above, the Tg of thepolymer) can be compared. Rather than measure NSR, however, it may bepreferable to make some other measurement such as volume of bottlesblown to the onset of strain hardening, which can be correlated to NSRby a simple mathematical relation.

The present invention also provides an apparatus for analyzing samplesof thermoplastic articles for a selected physical property in a mannersuch that consistent results are obtained which comprises

a) means for at least partially submerging the articles in a boilingliquid,

b) means for withdrawing the articles from the boiling liquid after theyhave been heated to at least their glass transition temperature,

c) means for stretching the thermoplastic articles at a temperature ofat least their glass transition temperature until the onset of strainhardening occurs, and

d) means for determining a desired property which is, or is related to,the expansion of the articles.

More particularly, apparatus is provided for analyzing samples ofthermoplastic bottle preforms of the same predetermined volume todetermine the amount of expansion at the onset of strain hardening in amanner such that comparable results are obtained on a consistent basiswhich comprises

a) a vessel for containing a liquid,

b) means for supplying heat to maintain the temperature of the liquid atits boiling point for the duration of analysis,

c) a movable member having means adapted to hold a bottle preform by itsneck, the moveable member having a first position for holding at least aportion of said bottle preform submerged in the boiling liquid, and asecond position for holding the bottle preform in a position outsidesaid boiling liquid,

d) means for expanding the bottle preforms until they reach the onset ofstrain hardening, and

e) means for determining the volumes of the expanded preforms forcomparing with the volume of a control sample.

According to the invention there is also provided a method ofdetermining the stretching characteristics of thermoplastic articleshaving a predetermined relation of natural stretch ratio to a selectedphysical property on a consistent basis which comprises the steps of

a) determining at least one initial dimension of a sample of the articleat a selected temperature (since the initial volume of preforms for aparticular test is constant, obtaining the NSR is unnecessary),

b) submerging the sample in a boiling liquid having a consistent boilingpoint, the boiling temperature of which is sufficient to heat the sampleat least to its glass transition temperature, for a time sufficient forthe article to reach its glass transition temperature across itsthickness but without attaining appreciable crystallization,

c) stretching the article in at least said initial dimension to theonset of strain hardening,

d) determining the stretched dimension of the article,

e) determining the ratio of the stretched dimension to the initialdimension, and

f) correlating the ratio determined in e) to a precalculated associationwith a selected physical property.

Also, according to this invention, there is provided a method ofdetermining the stretching characteristics of a thermoplastic containerhaving a predetermined relation of natural stretch ratio to a selectedphysical property on a consistent basis which comprises the steps of

a) determining the initial volume of the container at a selectedtemperature,

b) submerging the container in a boiling liquid having a consistentboiling point, the boiling temperature of which is sufficient to heatthe container to at least its glass transition temperature for asufficient time for the container to reach its glass transitiontemperature across its wall thickness but without attaining appreciablecrystallization,

c) injecting a gas into said container to pressurize it and blow it tothe onset of strain hardening,

d) determining the volume of said container after step c),

e) determining the ratio of the volume determined in d) with its initialvolume, and

f) correlating the ratio determined in step e) to a precalculatedrelationship with a selected physical property.

Further, according to this invention there is provided a method ofdetermining the stretching characteristics of a thermoplastic polyesterpreform from which a bottle is to be blown, the polyester preform havinga predetermined relation of natural stretch ratio to inherent viscosity,which comprises the steps of

a) determining the initial volume of a selected major portion of thepreform at a selected temperature,

b) submerging the selected major portion of said preform in a boilingliquid having a consistent boiling point, the boiling temperature ofwhich is sufficient to heat said preform to at least its glasstransition temperature for a time long enough for the selected majorportion to reach its glass transition temperature across its wallthickness but without attaining appreciable crystallization,

c) injecting a gas into said preform under sufficient pressure to blowit to the onset of strain hardening,

d) determining the volume of the blown preform after step c),

e) determining the ratio of the volume determined in d) with its initialvolume, and

f) correlating the ratio determined in step e) to a precalculatedrelationship with inherent viscosity.

In the case of biaxial orientation of stretch blown thermoplasticbottles, the NSR is defined as A2/A1, where:

A2=area of stretched surface at onset of strain hardening

A1=area of original (unstretched) surface

In the case of uniaxial orientation, e.g., thermoplastic fiber andfilament, the NSR is defined as L2/L1, where:

L2=length of stretched segment at the onset of strain hardening

L1=length of original (unstretched) segment

Thus, in accordance with the present invention, a boiling liquid,preferably water, is used to consistently heat the preforms to the sametemperature for each test. Although a variety of other liquids may beused, boiling water is preferred for a number of obvious reasons. It isessential that the free-blow testing be done using the same conditions(e.g., temperature, blow pressure, and rate of stretching) on aday-to-day basis. Since the preform temperature has a large effect onthe bottle volume, it is important that the method of heating be veryconsistent, and using boiling water offers that assurance. "Consistentboiling point" means that the liquid boils at a predeterminedtemperature consistently while at the same barometric pressure.

The temperature at which water boils is dependant upon altitude,barometric pressure, and water purity. However, the altitude will beconstant for a given location and the purity of the water should berelatively consistent. It is preferred that either distilled ordemineralized water be used. Variations in temperature due to changes inthe barometric pressure will typically be very slight.

While water is preferred, other liquids or mixtures of liquids such assome hydrocarbons, alcohols, ketones, and esters could also be used,e.g., 2-butanol, isobutyl alcohol, n-propyl alcohol, diethyl ketone,methyl propyl ketone, methyl isobutyl ketone, n-heptane, methylcyclohexane, and propyl acetate.

If a liquid other than water is used, the liquid should boil at atemperature of at least the Tg to soften the preform enough for it to befree-blown. The glass transition temperature of PET is about 80° C.However, the combination of temperature and heat transfer coefficientshould be such that the inside surface of the preform would reach Tgprior to the outside surface crystallizing, if the preform is heatedfrom the outside with the liquid. Obviously, heating may be done fromthe inside by filling the preform with the liquid, or by a combinationof inside/outside heating.

In a broad sense, the present invention may be used in connection witharticles such as sheet material, film and fibers as well as bottles. Insuch cases the film or sheet material may be unilaterally or bilaterallystretched, and fibers may be unilaterally stretched for determiningnatural stretch ratio. The Tg of the thermoplastic article should beattained across the thickness thereof. In the case of a preform, the Tgmust be reached across the wall thickness. However, the heating shouldbe at a temperature and time such that no appreciable crystallizationoccurs.

In the case of preforms, stretching is conveniently done by pressurizingand blowing the bottle until the onset of strain hardening. An inertgas, preferably air, is used for the pressurization. Other suitableinert gases will be apparent to those skilled in the art. In the case offibers, films and sheeting, stretching may be accomplished by meansknown to those skilled in the art. For example, pairs of driven niprolls, operated at selected different speeds may be used. In the case offibers, films and sheeting, dimensions may be measured initially, i.e.,before heating while the article is at a selected temperature in thesolid state, and then measured at the onset of strain hardening. Thedifference in these measurements can be calculated readily.

In bottles, a convenient way of relating these measurements is byvolume. It will be apparent to those skilled in the art that the samephysical property is being measured, i.e., amount of strain until theonset of strain hardening.

Referring first to FIG. 2, a stress-strain curve is illustrated which istypical of PET at a temperature of about 100° C.

In Region A, the stress rises rapidly and results in a certain amount ofstrain. The curve then flattens out in Region B, where the stressremains relatively constant while a large amount of strain occurs. Nextthe curve turns upward into Region C, the strain hardening region wherestrain induced crystallization takes place and the highest tensileproperties are achieved. The onset of strain hardening is the transitionregion between Regions B and C. The onset of strain hardening may bedetermined as follows: Extend lines from Regions B and C until theyintersect. Bisect the angle adjacent to the stress-strain curve, andextend that bisecting line to intersect the curve. The point at whichthat line and the curve intersect is the onset of strain-hardening.

Apparatus suitable for carrying out the present invention is illustratedin FIG. 1, which is a schematic elevation view of such apparatus.Referring to the drawings, double acting air cylinder 10 is rigidlymounted on base 12. Arm 14 of air cylinder 10 is adapted to be extended,as shown, or withdrawn, in which case bracket 16 would submerge preform18 in the container of boiling liquid 20. The water is heated by aheater 22, which may suitably be a hot plate. Preform 18 is providedwith threads during an injection molding process, as is well known tothose skilled in the art, and the threads allow it to be screwed intopreform holder 24. Preform holder 24 has associated therewith an airsource through line 26.

In operation, the liquid is heated to its boiling point. Air cylinder 10is actuated, placing the preform 18 in the boiling liquid. Preform 18 ismaintained in this position until heated sufficiently as describedherein. At that point, the air cylinder 10 extends arm 14 to carry thepreform out of the boiling liquid. The hot preform 18 is rotated awayfrom the boiling liquid, then blown to the onset of strain hardening andallowed to cool while still under pressure. The pressure is relieved andthe blown bottle 30 is removed for selected measurements (e.g., volume).

It was found that bottles could be free-blown to the NSR using variouscombinations of heating time and blow pressure (3 to 5 minutes and 50 to90 psi). As expected, the NSRs, and thus bottle volumes, weresignificantly different. However, for a particular combination of timeand pressure, the resulting NSRs and volumes were generally consistent.

From the standpoint of conditions on heating time and pressure, thefollowing points should be considered:

1. The length of time a preform needs to remain in the boiling liquidwill depend primarily on its thickness.

2. The length of time must be sufficient for the inside surface of thepreform to reach the glass transition temperature of the material.

3. Leaving the preform in the liquid long enough for the inside surfaceto reach a temperature significantly greater than 95° C. for PET willonly increase the cycle time. Preferably, the inside surface is 85° to95° C. for PET.

4. Leaving the preform in the liquid too long can result in its outersurface starting to crystallize which will effect the NSR.

5. The blow pressure needed will depend on preform thickness and onpreform temperature.

6. The pressure should be sufficient to fully orient all of the materialbelow the support ring.

7. The pressure should not induce enough stress to cause the bottle toburst.

8. Ideally, the pressure should be such that a very slight amount of"pearlescence" occurs. Pearlescence is a term commonly used in the PETbottle industry to describe a phenomenon that occurs when the onset ofstrain hardening is exceeded by about 5%. When that happens,microfractures occur, giving a hazy appearance with the sheen of anoyster pearl. Therefore, "pearlescence" is an excellent visualindication that the onset of strain hardening has been reached.

As an example, 200 grams of water were added to 250 pounds of PET (0.76IhV copolymer) and blended. Three days later, the pellets were dried at4 different sets of conditions, molded into 55-gram preforms, and thenfree-blown using the following conditions. Preforms from the same cavityof the injection mold were used.

1. With the water at boil, weigh the beaker of water and adjust it toinsure that the water level is at a predetermined level.

2. With the preform screwed into the holder, lower it into the boilingwater.

3. After 4 minutes, raise the preform and pivot it away from the beakerof boiling water.

4. Quickly wipe the preform dry. Drops of water on the preform-will actas a heat sink, and thus cool those areas, which in turn prevents themfrom stretching properly.

5. Twenty seconds after removal from the water, pressurize the preformwith 80 psig air. The inside diameter of the air line was about 0.18inch. That diameter has an effect on the rate at which the preform isinflated, and thus the rate at which the PET stretches. The ultimatesize of the free-blown bottle is somewhat dependant upon the stretchrate, with faster rates yielding smaller bottles.

6. Remove the bottle from the holder, and fill it completely with tapwater, and cap it, insuring there are no air bubbles present.

7. Weigh the filled bottle, subtract the weight of the bottle and capand record the net weight.

Six bottles were selected and cut for IhV testing (Table 2). A graph ofAverage Bottle Volume Vs. Average Bottle IhV is given in FIG. 3.

Furthermore, a 0.01 dl/g difference in IhV corresponds to a 75 ccdifference in the free-blown bottle volume, showing that the test issensitive enough to detect relatively small differences in IhV.

                  TABLE 1                                                         ______________________________________                                        Free-Blown Bottle Volumes                                                     (Cubic Centimeter)                                                            Drying                                                                        250° F. for                                                                      250° F. for                                                                        275° F. for                                                                      300° F. for                            4 Hr      5 Hr        6 Hr      6 Hr                                          ______________________________________                                        2915      2715        2595      2467                                          2841      2860        2552      2500                                          2855      2784        2531      2532                                          2952      2725        2614      2485                                          2874      2814        2672      2467                                          3078      2789        2550      2444                                          2956      2837        2539      2485                                          2834      2701        2565      2490                                          2959      2727        2633      2416                                          2842      2852        2611      2524                                          2973      2815        2630      2475                                          3010      2767        2635      2522                                          2970      2791        2567      2489                                          2944      2810        2585      2525                                          2948      2761        2654      2478                                          3068      2728        2601      2546                                          3017      2913        2648      2464                                          3082      2818        2593      2506                                          2987      2883        2610      2480                                          3044      2910        2588      2531                                          --        --          --        2537                                          2957      2800        2599      2490                                                                          2504                                                                          2546                                                                          2548                                                                          2440                                                                          2443                                                                          2586                                                                          2541                                                                          2562                                                                          --                                                                            2501                                          ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Bottle IhV Data (With Corresponding Volumes)                                  IhV  Vol.     IhV    Vol.   IhV  Vol.   IhV  Vol.                             ______________________________________                                        0.682                                                                              2915     0.691  2860   0.727                                                                              2595   0.734                                                                              2500                             0.666                                                                              3078     0.699  2725   0.724                                                                              2531   0.733                                                                              2532                             0.689                                                                              2834     0.693  2789   0.730                                                                              2614   0.746                                                                              2444                             0.693                                                                              2973     0.698  2701   0.726                                                                              2672   0.743                                                                              2490                             0.682                                                                              2970     0.705  2767   0.731                                                                              2539   0.744                                                                              2416                             0.677                                                                              2948     0.698  2810   0.725                                                                              2565   0.743                                                                              2546                             ______________________________________                                         Note:                                                                         IhV data are in dl/g and volumes in cc.                                  

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

I claim:
 1. System for analyzing samples of thermoplastic articles for aselected physical property in a manner such that consistent results areobtained which comprisesa) means for at least partially submerging saidarticles in a boiling liquid, b) means for withdrawing said articlesfrom said boiling liquid after they have been heated to at least theirglass transition temperature, c) means for stretching said thermoplasticarticles at a temperature of at least their glass transition temperatureuntil the onset of strain hardening occurs, whereby a desired propertywhich is, or is related to, the elongation of said articles may bedetermined.
 2. System for analyzing samples of thermoplastic bottlepreforms of the same predetermined volumes to determine the amount ofexpansion of their natural stretch ratio in a manner such thatcomparable results are obtained on a consistent basis which comprisesa)a vessel for containing a liquid, b) means for supplying heat tomaintain the temperature of said liquid at its boiling point for theduration of analysis, c) a movable member having means adapted to hold abottle preform by its neck, said movable member having a first positionfor holding at least a portion of said bottle preform submerged in saidboiling liquid, and a second position for holding said bottle preform ina position outside said boiling liquid, d) means for expanding saidbottle preforms until they reach their onset of strain hardening,whereby the volumes of the expanded preforms may be determined forcomparing with the volume of a control sample.